A well treatment fluid for use in a subterranean formation penetrated by a wellbore is generally prepared by mixing a base fluid and a viscosifying agent. The base fluid is made up of an aqueous fluid and can be of any convenient source, including, but not limited to fresh water, seawater, natural brine and formulated brines such as 2% KCl solution. The viscosifying agent thickens or viscosifies the base fluid and may assist in the transport of solid particles in the fluid. A typical viscosifying agent includes or is made from a polysaccharide.
Polysaccharides have been used to alter the viscosity and rheology of aqueous solutions and are typically used in drilling, completion, and remedial operations. Among other uses, polysaccharides may be used as a part of fracturing gels for hydraulic fracturing, to viscosify drilling fluids, to control fluid loss, as blocking gels, as a part of gravel packing, frac pack fluids, etc. Heteropolysaccharides, in particular, have been used to alter the viscosity and other rheological characteristics of aqueous solutions as well as for secondary functions, such as emulsification, suspension of solids, stabilization, flocculation, etc. See, for example, the many applications of heteropolysaccharides in U.S. Pat. No. 4,326,052 filed Aug. 14, 1990, and U.S. Pat. No. 4,401,760 filed Oct. 21, 1981, both assigned to Merck & Co., Inc, which are hereby incorporated by reference in their entirety.
However, a treatment fluid can suffer a complete loss of viscosity under certain conditions to which it may sometimes be subjected in treating a subterranean formation. Examples of such conditions include high shear (caused by the pumping and placement), high temperatures, high pressure, high salinity, and low pH. Under such conditions, the polymeric material used to viscosify the treatment fluid may degrade rather quickly and lose its viscosifying properties.
In fact, many heteropolysaccharides are ineffective at temperatures above 80° C. Xanthan, in particular, is commonly used to viscosify treatment fluids for applications such as gravel packing, but becomes less effective above 80-90° C.
In order to maintain reservoir fluid control and formation/fluid compatibility, the density of the fluid may be controlled by addition of soluble materials such as salts. These salts can also contribute to loss in viscosity of the treatment fluid.
Accordingly, there is a need for a viscosifying agent that provides high stability in a treatment fluid at elevated temperatures in solutions containing electrolytes.